Determination of cavity size in earth formations penetrated by a borehole



INVENTOR.

JOSEPH A. CALDWELL ATTORNEY- Q d v., www

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ATTORN EY.

United States Patent Olice 3,659,469 Patented Oct. 23, 1962DETERMINATION F CAVITY SIZE DI EARTH FORMA'IIUNS PENE'IR'IED BY ABQREHGLE Joseph A. Caldwell, Houston, Tex., assignor, by mesneassignments, to Jersey Production Research Company,

Tulsa, Okla., a corporation of Delaware Filed June 12, 1961, Ser. No.116,532 4 Claims. (Cl. 73--149) This invention relates generally to welloperations, such as sand consolidation operations, and more particularlyto determination of the presence of and size of cavities that may existaround boreholes through which earth formation fluids are produced.

There are a number of circumstances under which it is desirable todetect and determine the size of cavities in earth formations. Suchoperations are particularly desirable in connection with consolidationof loose sands from which hydrocarbons are produced. When hydrocarbonsare produced from loose sands, it is usual that a substantial amount ofsand will iind its way through perforations in the casing pipe stringusually lining the wall of a borehole. This sand either will accumulateat the bottom of the borehole or will be produced with the hydrocarbonsor other earth formation fluids. In the latter case, substantial damagemay be done tollow tubing pumps and other well apparatus.

It is known to consolidate loose sands by injecting plastic or resinoussand consolidating compositions into the sands to bind together the sandgrains while leaving pore spaces through which earth formation fluidscan flow. Not all attempts at consolidating sands with resinous orplastic compositions have been successful. A reason for such failures isbelieved to be as described below. If an unsuspected cavity shouldsurround the borehole in the earth formation, an insutlicient amount ofthe sand consolidating composition will be pumped into theearthformation. Should the amount of the sand consolidating compositionbe insufcient to even fill the cavity, manifestly a considerable portionof the formation around the upper part of the cavity will not becontacted by the consolidating composition. Furthermore, if theexistence of the cavity `should be suspected and too much of theconsolidating composition should be pumped into the well, the hardeningcatalyst that usually follows the consolidating composition will notcontact the earth formation to harden the consolidating compositionbefore formation fluids are produced again from the sand. The resultwill be the same as if no attempt at all were made to consolidate thesand, and the consolidation attempt will be a failure.

In accordance with the teachings of the present invention, use is madeof a testing liquid that is incapable of penetrating the earth formationcontaining the cavity and that is immiscible with other fluids that maybe present in the Well. The liquid is injected through a pipe string ofknown dimensions into the borehole at known ilow rate to a level atwhich it is believed a cavity exists. Fluid in the borehole before theliquid is injected thereinto is forced ahead of the liquid into an earthformation containing the cavity. In the event that it is not known forsure whether the fluid forced ahead of the liquid is capable ofpenetrating the formation, the fluid is displaced with a fluid that ispositively known to be able to penetrate the formation and to beimmiscible with the testing liquid. The testing liquid is injected untila sharp pressure rise is obtained which indicates that no more iluid canbe forced into the formation and that the testing liquid entirely llllsthe cavity. Since the dimensions lof the pipe string are known, thedifference between the volume of the testing liquid pumped into the wellas described above and the volume of the pipe string will give thevolume of the cavity.

Objects and features of the invention not apparent from the abovedescription will become evident upon consideration of the followingdescriptive matter taken in connection with the accompanying drawings,wherein:

FIGS. l and 2. are schematic representations of well installationsillustrating steps in the performance of the invention;

FIG. 3 is a schematic representation similar to FIG. 2 illustrating amodification of the invention; and

FIG. 4 is a graph of testing liquid pumping pressure as a function oftime, which graph is useful in understanding the invention.

With reference now to FIGS. 1 and 2, there is shown a well installationincluding a borehole 24 in the earth that penetrates a hydrocarbon earthformation 35 containing a cavity 41. A `casing pipe string 23 inborehole 24 is cemented to the side of the borehole in the usual mannerby a cement sheath 25. The casing and cement sheath are assumed to havebeen perforated so as to produce perforations 43 which provide iluidcommunication between productive earth formation 35 and the interior ofthe casing. The -flow tubing 15 is suspended in the casing string 23 inthe usual manner by wellhead apparatus 21.

It will be assumed that the Well has been produced for a time intervalsutlicient to cause a substantial amount of sand to flow into thecasing, as the result of which cavity 41 is formed in earth formation35. A substantial amount of sand 45 is shown as having accumulated inthe bottom of the casing 23.

The usual tlow lines `17 and 14 are respectively connected to thewellhead so as to respectively provide communication with thetubingcasing annulus andthe bore of the tubing. The lines 17 and 14 arerespectively controlled by valves 19 and v13. A liquid pump 5 having asuction line 3 and an exhaust or pressure line 11 is shown as beingprovided with a pressure gauge 9. The exhaust line 11 is connected toline 14, and the suction line 3 is connected to a source of liquid whichwill be described below.

Reference numeral 29 designates well bore iluids that were found to beoriginally present in the well. The fluids may be drilling fluids, earthformation iluids, testing liquids previously injected into the well, orcombinations thereof. If it is desirable to remove such fluids from thetubing and the lower part of the well bore, crude oil 33 is pumped intothe tubing string by pump 5, and the originally present lluids arecirculated up the tubing-casing annulus and out line 17. Only a portionof the iluid 29 need be so circulated as shown in FIGS. l and 2. Thelower portion of the tubing should be immersed in crude oil. Wellheadvalve 19 is closed after the desired amount of iluid is removed.

A low filtration rate testing liquid 31 is now pumped into tubing string15. Preferred compositions for this testing liquid 31 will be describedbelow. The oil 33 is forced ahead of the testing liquid into the earthformation 35 as indicated by arrows 42. The liquid 29 in the upperportion of tubing-casing annulus will be virtually unaffected by theiluid llow and will remain in the annulus except at the lower end oftubing 15, as shown in FIG. 2. The low filtration rate liquid 31 willflow down the tubing 15, into the casing bore below the lower end of thetubing 15 through perforations 43, and into the cavity 41. As thetesting liquid 31 is pumped into the well and before it enters thecavity, the pressure noted by gauge 9 will remain at a relatively lowvalue over the time interval from time zero to time T1 as indicated onthe graph of FIG. 4. During the interval that the cavity is filling withtesting liquid (i.e., during the time interval from time T1 to time T2),the pressure will gradually rise as more and more of the testing liquidnds its way to the cavity-formation interface. At time T2 the cavitywill be substantially filled with testing liquid and a sharp pressurerise will be noted as the flow rate of testing liquid sharply drops off.During this entire operation the flow rate of testing liquid preferablyis kept constant, but under all circumstances the total volume oftesting liquid is measured by a suitable ilow meter 2 in the line 3.Inasmuch as the dimensions of the tubing and casing Z3 are known, thevolume of testing liquid within tubing 15 and the portion of casing 23below the lower end of tubing 15 also will be known. By subtracting thisknown volume from the 'volume of testing liquid injected into tubing 15during the time interval from time zero to time T2, the volume of cavity41 can be very accurately determined.

The embodiment of the invention shown in FIG. 3 differs from that shownabove in that the liquid originally used to remove fluids originally inthe borehole consists of water, preferably salt water. The water isinjected into the tubing-casing annulus by connecting exhaust line 11 toline 17. The water displaces the original lluids 29 either entirely orto a suitable distance up the tubing 15. The water is then followed withan oil-based testing liquid 32 which, like the liquid 31, ischaracterized by a low filtration rate. The reason that the liquids mustbe injected into the tubing-casing annulus is that the testing liquid 32would bubble up through the salt water 34 if the procedure shown in FIG.1 were followed, thus resulting in an inaccurate determination of cavitysize. Otherwise, the steps followed in the embodiment of FIG. 3 aresubstantially the sa-me as the steps followed in the embodimentdescribed with respect to FIGS. l and 2.

The testing liquid 31 used in the embodiment described with respect toFIGS. l and 2 is immiscible with the liquid pumped thereahead.Preferably, the liquid 31 consists of any of the following: 1 to 10 lbs.of carboxy methyl cellulose to l bbl. of water; 1 to 10 lbs. of anorganic gum such as guar gum or gum tragacanth, to 1 bbl. of water; or 1to l0 lbs. of gelatinized starch to 1 bbl. of water. The testing liquid32 used in the embodiment of FIG. 3 may consist of 0.5 to 5 parts of ametal soap such as napalm (a mixture of aluminium palmonate and aluminumoleate) to `100 parts of oil, or 0.5 to 5.0 parts of partiallypolymerized butadiene to l0() parts of oil. Other suitable mixtures maybe used.

Testing fluids that can be used both in the technique described withreference to FIGS. 1 and 2 and in the technique described with referenceto FIG. 3 are the oil-inlwater emulsions described in U.S. Patent2,805,722-Priest et al. These oil-in-water emulsions are emulsied bynonionic and anionic agents such as polyoxyethylene sorbitan monolaurateand a film strengthening agent such as sulfonate phenol formaldehydepolymer, sodium carboxy methyl cellulose, and sodium lignosulfonate. Thedensity of the emulsion can be controlled by using soluble salts such assodium or calcium chloride or high density liquids such astetrachlorethane.

In the practice of the invention, it will be determined that on occasionthe fluids originally in the well are suficiently immiscible with thetesting fluids 31 and 32 and are suiciently capable of penetratingformation 35 to be used with the invention. In such a case, there willbe no necessity of pumping fluids 33 and 34 into the wellhead of thetesting liquid. However, it is usually found that earth formation fluidsconsist not of oil or of water but mixtures of oil and water, and shouldbe displaced from the flow path of the testing liquids before thetesting liquids are injected into the well.

While the invention has been described with respect to sandconsolidating operation, it is manifest that it can be used in other oilwell operations such as cementing, fracturing, and acidizing.Furthermore, apparatus and interconnections of apparatus may be usedother than that described above.

What is claimed is:

l. A method of measuring the total volume of an earth formation cavityand Well pipe means of known volume disposed in a borehole penetratingthe cavity for the purpose of obtaining the volume of the cavity,comprising: forcing fluids in the well pipe means and the cavity intothe earth formation by pumping through the Well pipe means at known flowrate a fluid incapable of penetrating the formation while continuouslymeasuring the lluid pressure; and discontinuing pumping when a sharpincrease in pressure is obtained.

2. A method of measuring the total volume of an earth formation cavityand well pipe means of known volume disposed in a borehole penetratingthe cavity for the purpose of obtaining the volume of the cavity,comprising:

displacing fluid initially in the well pipe means with lluid known to becapable of readily penetrating the earth formation containing thecavity; forcing the fluid capable of penetrating the earth formationinto the earth formation by pumping through the well pipe means at knownflow rate a fluid incapable of penetrating the formation whilecontinuously measuring the fluid pressure; and discontinuing pumpingwhen a sharp increase in pressure is obtained.

3. In a method of measuring the volume of a cavity in an earth formationpenetrated by a well bore having therein a rst fluid adapted to beforced into the formation, the improvement comprising: pumping at knownilow rate into the cavity through well pipe means of known volumeextending to the depth of the cavity, a second fluid substantiallyincapable of penetrating the formation to force the first iluid into theformation; and discontinuing pumping when a sharp rise in pressureoccurs.

4. A method of measuring the volume of a cavity in an earth formationpenetrated by a well bore cased and tubed with tubing and perforatedcasing of known dimensions, comprising: circulating down tubing and atleast part way up the tubing-casing annulus to a level above vthecavity, a liquid adapted to penetrate into the earth formationcontaining the cavity to be measured; forming a. second iluid immisciblewith the first lluid and incapable of appreciably penetrating into `saidearth formation with the casing bore closed, pumping said second fluidinto the tubing at known flow rate to force said first iluid into theformation until a sharp pressure rise indicates that the cavity issubstantially filled With said second fluid, whereby the volume occupiedby said second fluid in the tubing and in `the casing adjacent theformation may be subtracted from the measured volume of said secondfluid pumped into the well bore to determine the volume of the cavity.

McConnell Mar. 18, 1941 Huntington Mar. 23, 1943

